This application is based on Patent Application No. 2001-58828 filed Mar. 2, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a wide-bandwidth optical fiber coupler with less polarization dependency loss and less excessive loss, and a method and an apparatus for easily and stably fabricating the optical fiber coupler.
2. Description of the Related Art
An optical coupler with less wavelength dependency, e.g. a wide-bandwidth optical fiber coupler for branching and coupling a wide-bandwidth optical signal may enjoy wide utilization as various optical parts for use in general optical fields and micro optical fields and in optical communication and optical data processing.
Upon fabrication of such a wide-bandwidth optical fiber coupler, there is known a prior art method wherein one of two optical fibers possessing the same structural parameters of the optical fibers is previously heated and drawn to force the structural parameters of these two optical fibers to be different or two optical fibers possessing mutually different structural parameters of the optical fibers are used, melted and drawn in a twisted state of the fibers.
Referring to FIGS. 29 to 31, fabrication procedures of such a prior art wide-bandwidth optical fiber coupler is illustrated in schematically. Opposite ends of one optical fiber 1 is held on a pair of carrying blocks 2. A coating is removed from a central portion of the optical fibers 1 located between these carrying blocks 2 as illustrated in FIG. 29. A distance between the pair of the carrying blocks 2 is increased while heating an exposed optical fiber strand 3. The portion of the optical fiber strand 3 is preliminarily drawn, as illustrated in FIG. 30. There are twisted a preliminarily drawn optical fiber 5 and the optical fiber 1 which optical fiber 1 is not preliminarily drawn and in which a coating thereof is only removed from a central portion thereof are twisted along that portions of the optical fiber strands. In this state, both ends of these optical fibers 1 and 5 are again held on the pair of the carrying blocks 2. The distance between the pair of the carrying blocks 2 are increased while again heating the portions of the twisted optical fiber strands 3 with the burner 4, as illustrated in FIG. 31. The two optical fiber strands 3 are hereby melted and drawn at those twisted portions to obtain such an optical fiber coupler 6 as illustrated in FIG. 32.
When a wide-bandwidth optical fiber coupler 6 with reduced wavelength dependency is fabricated, the foregoing prior art method where there are used two optical fibers with mutually different structural parameters requires that two kinds of optical fibers previously should be fabricated. This is not general in view of the production cost.
To solve this difficulty, the following method is exclusively employed: two optical fibers of the same kind are prepared, one of which is preliminarily drawn to change the parameters concerning the structure.
The prior art method illustrated in FIGS. 29 to 31, however, needs to preliminarily draw the optical fiber strand 3 by about 10 mm using the burner 4. In the method, it is essentially impossible to finely control heating temperature for the optical fiber strand 3 in the unit of several tens of xc2x0 C. for example so as to prevent a wire diameter from being varied. In the prior art method, the wire diameter of a portion of the preliminarily drawn optical fiber strand 3 is narrowed, so that it is necessary to mutually twist the preliminarily drawn optical fiber strand 3 and the optical fiber strand 3 of the optical fiber 1 from which sheath is only simply removed for securely bringing them into close contact. As illustrated in FIG. 23, when the two optical fibers 1 and 5 are mutually twisted, it is necessary to apply tension uniformly to those two optical fibers 1 and 5, so that those fibers should be twisted with subtle discretion. The prior art method therefore suffers from difficulty that work is complicated with deteriorated reproducibility. The prior art method further suffers from a difficulty that the optical fibers 1 and 5 are subjected to twisting, so that the optical fiber strand 3 suffers from larger polarization dependency owing to internal stress produced in itself, and allows polarization dependency loss (hereinafter, may be simply referred to PDL.) to be increased together with increased excessive loss.
Referring to FIG. 33, there is illustrated wavelength dependency of a coupling ratio, i.e. a branch ratio between a symmetrical optical fiber coupler A where an optical coupling portion is configured symmetrically and asymmetrical wide band optical fiber couplers B and C where an optical coupling portion is not symmetrical). For such a branch ratio of a general symmetrical optical fiber coupler in which parameters concerning the structures of two optical fibers constituting an optical fiber coupler A are same for each other, it varies periodically from 0 to 100% owing to the wavelength of an optical signal. In contrast, for wide band optical fiber couplers B and C possessing different parameters concerning the structures of two optical fibers constituting an optical coupling portion, a branch ratio is reduced to a specific value, 100% or less by combining optical fibers with outer diameters of clad portions thereof for example being respectively 115 xcexcm and 125 xcexcm. There is thereupon utilized a flat portion located in the vicinity of the maximum of the just-mentioned branch ratio. As illustrated in FIG. 33, the wide-bandwidth optical fiber coupler B has the branch ratio of 50% at wavelength of 1.4 xcexcm and the wide-bandwidth optical fiber coupler C has the optical branch ratio of 20% at wavelength of 1.4 xcexcm. It can be understood therefrom that a change in the branch ratio in the vicinity of the maximum branch ratio is more flattened than the wavelength characteristic of the symmetrical optical fiber coupler A would be.
For fabricating the aforementioned wide-bandwidth optical fiber coupler there is known a method proposed by International Application PCT/GB 86/00445. More specifically, one single mode optical fiber is preliminarily drawn, and the preliminarily drawn optical fiber and another optical fiber not preliminarily drawn are combined, melted and drawn to successfully obtain a wide-bandwidth optical fiber coupler 6 as illustrated in FIG. 34 and FIG. 35 which illustrates a cross sectional structure viewed along XXXVxe2x80x94XXXV shown in FIG. 34. In the resulting optical fiber coupler 6, a propagation constant of the one single mode optical fiber 1 is altered with the aid of preliminary drawing to obtain different parameters from those concerning the structure of the optical fiber 5 not preliminarily drawn with the maximum branch ratio brought into a specific value less than 100%, and a flat wavelength characteristic in the vicinity of the maximum value of the branch ratio is utilized. In FIG. 34, designated at 7a is a core portion, 7b is a clad portion, 8 is preliminarily drawn portion, and 9 is a melted drawn portion.
The aforementioned prior art fabrication method for the wide-bandwidth optical fiber coupler 6 using the preliminary drawing, however, has a difficulty that uniform preliminary drawing is difficult. More specifically, uniform heating control for the preliminary drawn portion 8 is difficult to make impossible precision configuration control and hence to make it difficult to obtain a uniform outer diameter of the melted drawn portion 9. The optical fibers 1 and 5 might thereupon been sometimes bent upon processing the resulting wide-bandwidth optical fiber coupler 6. This might cause set value control for the branch ratio to be difficult and therefore accuracy of flatness of the branch ratio of the drawn optical fiber to be unsatisfactory with the very bad yield of the coupler.
To solve the aforementioned difficulty with the preliminary drawing described above, Japanese Patent Application Laid-Open No. 2-171705 discloses a method wherein two optical fibers possessing mutually different propagation constants are drawn with the mutually equal drawing, and are then melted and further drawn. Further, Japanese Patent Application Laid-Open No. 2-259704 discloses another method wherein between two single mode optical fibers including clad portions thereof having mutually different diameters thereof the one optical fiber having the same outer diameter as that of the same optical fiber at the other end is previously melted and connected with both ends of the other optical fiber, and then the two optical fibers are melted and drawn.
Also in these two methods, however, structural parameters of the two optical fibers are made asymmetrical and the maximum value of the branch ratio is brought to a specific value less than 100%, and wavelength flatness in the vicinity of the maximum value is utilized. The degree of melting is still high so as to provide close coupling.
When two optical fibers having the same structural parameters are mutually melted and drawn to fabricate a symmetrical optical fiber coupler, the maximum value of the branch ratio thereof becomes 100% as illustrated in FIG. 33. Thereupon, it is contemplated from the viewpoint of the neighborhood of the maximum value of the branch ratio being used that a wide-bandwidth optical fiber coupler having an arbitrary branch ratio can not been fabricated. Accordingly, a wide-bandwidth optical fiber coupler is conventionally fabricated by preparing two optical fibers having mutually different structural parameters such as a diameter of a clad portion or combining a preliminary drawn optical fiber and a not preliminarily drawn optical fiber.
In optical fiber communication, there are required wide-bandwidth optical fiber couplers possessing various branch ratios not only of 50% but also of 20, 10, 5, 2%, etc. Further, each time a branch ratio is altered, there must be prepared optical fibers having different structural parameters such as a core diameter, a specific refractive index, and a clad outer diameter, resulting in the high fabrication cost. When a wide-bandwidth optical fiber coupler fabricated with a different clad diameter optical fiber is assembled in an optical communication network, it is necessary to connect a different clad diameter optical fiber with opposite ends of the assembled optical fiber. It is generally not easy technically to connect optical fibers possessing different parameters concerning such a structure mutually in series, resulting in the costing-up of fiber fabrication.
In view of the above description it is an object of the present invention to provide a method and an apparatus capable of easily fabricating with excellent reproduction a wide-bandwidth optical fiber coupler that possesses less polarization dependency loss and less excessive loss.
It is another object of the present invention to provide an optical fiber coupler that eliminates a preliminary drawing process, and a method capable of inexpensively fabricating the foregoing optical fiber coupler.
A first aspect of the present invention is an optical fiber coupler which comprises two optical fibers each including a core section for serving to transmit light and a clad section surrounding the former core section, the two optical fibers extending substantially parallel in a same flat plane, and a melting section where the clad sections of the two optical fibers are mutually melted substantially in a line contact state.
The present invention is grounded on the appreciation that the wavelength dependency of the branch ratio of the optical fiber coupler with low melting rate tends to become less than that of the branch ratio of the optical fiber coupler with high melting rate, as shown in FIG. 36.
In the optical fiber coupler according to the first aspect of the present invention, outer diameters of clad portions of two optical fiber couplers may be substantially equal to each other. In this case, even if a coupling rate of the optical fiber coupler is 100%, the optical fiber coupler with arbitrary branch ratio can be provided by making use of a portion with a mild modulation of the branch ratio.
It is advantageous that the branch ratio of the optical fiber coupler increases substantially monotonically in response to the wavelength of light that propagates in an optical fiber, and in the wavelength range of the light of from 1.3 xcexcm to 1.55 xcexcm the amount of a change in the branch ratio effectively lies within 20%.
In accordance with the first aspect of the present invention, in the wavelength range of light of from 1.3 xcexcm to 1.55 xcexcm the branch ratio of the optical fiber coupler may be 1 to 20%.
Outer diameters of clad portions of two optical fibers may be mutually different.
It is preferable that when the size of the maximum width size of the melting portion is assumed to be W, and outer diameters of the clad portions of the two optical fibers in the melting portion are assumed d1 and d2 respectively, a melting rate C represented by
C=[1xe2x88x92{W/(d1+d2)}]xc3x97100 
lies in the range of from 0.5 to 10%, preferably 1 to 7% which range corresponds to xe2x80x9csubstantially line contact statexe2x80x9d in the present invention.
A second aspect of the present invention is a method for fabricating an optical fiber coupler which comprises the steps of arranging mutually in parallel two optical fibers each including a core portion serving to propagate light therethrough and a clad portion surrounding the core portion to bring at least parts of the clad portions into close contact, melting mutually parts of the clad portions of the two optical fibers substantially in a line contact state by heating these clad portions in a mutual close contact state of at least parts of the clad portions, and heating and drawing the mutually melted two optical fibers.
In the method for fabricating the optical fiber coupler according to the present invention, the step of mutually melting the clad portions of the two optical fibers may include a step of heating the clad portions to 1500xc2x0 C. or higher.
The step of heating and drawing of the two optical fibers mutually melted may further comprise the steps of lowering heating temperature with respect to the two optical fibers after those optical fibers are melted, drawing melted portions of the two optical fibers in the state where the heating temperature is lowered, forcing monitor light to impinge from a one end side of any of the two optical fibers and detecting the monitor light from the other end side of at least one of the two optical fibers to measure a branch ratio thereof, and interrupting the drawing of the melted portions of the two optical fibers at the time when the branch ratio reaches a predetermined value. Therefore, an optical fiber coupler having a desired branch ratio can be obtained.
It is preferable that when the size of the maximum width size of the melting portion is assumed to be W, and outer diameters of the clad portions of the two optical fibers in the melting portion are assumed d1 and d2 respectively, a melting rate C represented by
C=[1xe2x88x92{W/(d1+d2)}]xc3x97100 
lies in the range of from 0.5 to 10%, preferably 1 to 7% which range corresponds to xe2x80x9csubstantially line contact statexe2x80x9d in the present invention. Therefore, the optical fiber coupler possessing excellent characteristic can be obtained.
For the heating for the optical fiber an electric ceramic microheater may be preferably employed in view of the ease of temperature control. It is then ensured that heating temperature for the optical fiber becomes accurately controllable to smoothly change the diameter of the optical fiber strand as well as successfully obtain a long drawn region possessing a uniform wire diameter. This assures fabrication of an optical fiber coupler possessing excellent characteristics. It is additionally possible to preliminarily draw, in the state where two optical fibers are previously arranged in parallel very closely, only the one optical fiber.
There may be substantially equal outer diameters of the clad portions of the two optical fibers where at least parts of the clad portions are brought into contact with each other.
The two optical fibers where at least parts of the clad portions are brought into contact with each other may be different from each other in their structural parameters. Therefore, it becomes possible to melt such two optical fibers without twisting them, whereby a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably.
In this case, it is possible to further provide a step in which only one of the two optical fibers are preliminarily drawn to bring structural parameters thereof to different ones. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.
The method for fabricating an optical fiber coupler may further comprise the steps of arranging mutually in parallel two optical fibers having the same structural parameter, and preliminarily drawing only one of the two optical fibers to provide mutually different structural parameters thereof. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.
In this case, the step of preliminarily drawing one optical fiber may include the steps of heating both the two optical fibers to distortion eliminating temperature or higher, and drawing only one of the heated two optical fibers. Otherwise, the same step may be performed at a heating temperature where one optical fiber does not melt with the other optical fiber. Therefore, it becomes possible to previously hold the two optical fibers closely and hence improve workability whereby an optical fiber coupler effectively ensured.
A third aspect of the present invention is an apparatus for fabricating an optical fiber coupler which comprises a pair of fiber carrying blocks for holding longitudinal opposite sides of two optical fibers possessing mutually different structural parameters, fiber fixing means provided on the fiber carrying blocks for fixing the two optical fibers to the fiber carrying blocks, a base for carrying the pair of the fiber carrying blocks movably longitudinally of the optical fibers, carrying block drive means for moving the pair of the fiber carrying blocks mutually oppositely in the opposite directions of the fiber carrying blocks, fiber forcing means provided on the fiber carrying blocks for forcing the two optical fibers such that parts of portions of the optical fibers from which coatings of the two optical fibers are removed and brought into contact with each other, and a heater mounted on the base movably in the direction intersecting the longitudinal direction of the two fibers along a flat plane containing the two optical fibers for heating the two optical fibers.
In accordance with the third aspect of the present invention, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.
A fourth aspect of the present invention is an apparatus for fabricating an optical fiber coupler which comprises a pair of first fiber carrying blocks for holding longitudinal opposite sides of a first optical fiber, first fiber fixing means provided on the pair of the first fiber carrying blocks to fix the first optical fiber to the fiber carrying blocks, a pair of second fiber carrying blocks for holding a second optical fiber at longitudinal opposite sides thereof in parallel with the first optical fiber, second fiber fixing means provided on the pair of the second fiber carrying blocks for fixing the second optical fiber to the second fiber carrying blocks, a base for movably carrying the first and second fiber carrying blocks longitudinally of the optical fiber, first carrying block drive means for mutually oppositely moving the pair of the first fiber carrying blocks in opposite direction thereof, second carrying block drive means for mutually oppositely moving the pair of the first fiber carrying blocks in the opposite direction thereof, fiber biasing means provided on the first and second fiber carrying blocks for biasing the first and second optical fibers such that parts of portions of the first and second optical fibers where coatings thereof are removed make contact with each other, a heater mounted movably in a direction intersecting a longitudinal direction of the optical fibers along a flat plane containing the first and second optical fibers for heating the first and second optical fibers, and heater movement means for driving the heating means in the direction intersecting the longitudinal direction of these optical fibers along a flat plane containing the first and second optical fibers.
In accordance with the fourth aspect of the present invention, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.
In the apparatus for fabricating an optical fiber coupler according to the third or fourth aspect of the present invention, there can be further provided a pair of coupling means for respectively integrally coupling first and second fiber carrying blocks located mutually closely. Therefore, when two optical fibers are heated, melted, and drawn, it becomes possible to securely integrally move them.
The fiber forcing means may include a fixing pin fixed to any one of the first and second fiber carrying blocks, a plunger disposed oppositely to the fixing pin between the first and second optical fibers and being movable oppositely to the fixing pin, and plunger fixing means for fixing the plunger at a predetermined position oppositely to the fixing pin. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.